Wheel/rail adhesion enhancement

ABSTRACT

If the wheel/rail adhesion for a railway vehicle is insufficient then the wheels may slip when braking. If slipping is detected, a control system may reduce the brake pressure to permit a controlled level of slip and so to optimize the braking force for the available adhesion. By determining the pressure supplied to the brake cylinder, a signal may be obtained which indicates the value of the adhesion. Alternatively the adhesion may be monitored by detecting any discrepancy between the braking deceleration demanded by the driver and the observed deceleration. The adhesion signal may activate a warning. It may also be used to adjust the rate at which sand is supplied by a sander to the wheel/rail interface. The rate at which sand is supplied may also be adjusted in accordance with other parameters such as the train speed.

This invention relates to a method and an apparatus for determining thewheel/rail adhesion for a railway vehicle on a railway track, and forenhancing that adhesion.

If the wheel/rail adhesion for a railway vehicle is insufficient, thenthe wheels may slip when the vehicle attempts to accelerate or to brake.This has been a recognised problem for many years, and the applicationof sand onto the rails has long been used to enhance adhesion. The sandmay be applied manually, or it may be supplied using compressed air, forexample in response to a measured value of the deceleration of thewheels (as in GB 1 349 145 (Girling)). It has also been suggested, in EP0 217 636 (Mitsubishi), that the quantity of sand particles dispensedmay be adjusted in accordance with the speed of the vehicle. Analternative way of dealing with this problem during braking is tomonitor both vehicle speed and wheel speeds, and if slipping is detectedto reduce the brake pressure, for example to permit a controlled levelof slip; for example the wheels might be allowed to slip so that theyturn at a rate say 10 or 15 percent less than that corresponding to thevehicle's speed. Such wheel slide protection systems are currently inuse. Controlled slipping of the wheels optimises the braking force, andalso tends to remove contamination from the rail head.

According to the present invention there is provided a method fordetermining the wheel/rail adhesion for a railway vehicle, the methodcomprising monitoring operation of the braking system when the brakesare applied, and hence determining the value of the adhesion.

In one embodiment the vehicle incorporates an accelerometer, and themethod comprises comparing the deceleration measured by theaccelerometer to the deceleration demanded by the driver, anddetermining from any discrepancy between those values of decelerationthe value of the adhesion. In an alternative embodiment the vehicleincorporates a wheel slide protection system which modulates thepressure in a brake cylinder in response to detection of wheel slip, andthe method comprises determining the pressure supplied to the brakecylinder, and determining from that pressure the value of the adhesion.

The value of adhesion may be indicated to the driver of the vehicle. Ifexceptionally low adhesion is identified in this way, a warning messagemay be transmitted to other vehicles on the line, and to signallingstaff. Where the method involves determining the pressure supplied tothe brake cylinder, the pressure may be measured, or alternatively maybe calculated, for example by a computer in the wheel slide protectionsystem.

The present invention also provides a method for enhancing thewheel/rail adhesion for a railway vehicle by applying adhesion-enhancingparticles to the wheel/rail interface at a rate which is adjusted inaccordance with the value of the adhesion so determined.

The adhesion-enhancing particles may be particles of sand or quartz orother suitable material and may be supplied dry, using compressed air.The rate at which the particles are supplied may also be adjusted inaccordance with other measured values, in particular the vehicle'sspeed, and preferably also the vehicle's deceleration. Furthermore therate at which particles are supplied may also be adjusted in accordancewith the desired deceleration, as specified by the driver. Currently inthe United Kingdom trains which dispense sand do so at a standard rateof 2 kg/min. Adjusting the rate of supply of particles in accordancewith vehicle speed to higher values than that, at higher speeds (sayabove 50 km/hr), will enable improved braking performance to beachieved, while reducing the rate of supply to lower values than that,at low speeds (say below 15 km/hr), will minimise the problems caused tothe track circuit.

The invention also provides an apparatus for enhancing the wheel/railadhesion for a railway vehicle, by adjusting the rate of supply ofadhesion-enhancing particles in the manner defined above. The rate ofsupply of particles may be continuously variable, or alternatively therate of supply may be adjustable in steps. It will be appreciated thatany such apparatus is desirably arranged so that if the adjustmentfails, the apparatus should revert to dispensing the particles at aconstant rate (e.g. 2 kg/min).

The invention will now be further and more particularly described, byway of example only, and with reference to the accompanying drawingwhich shows a flow diagram for a train braking system.

BRIEF DESCRIPTION OF THE DRAWING

The drawing is a flow diagram and illustrates a preferred embodiment ofa braking system for a train.

Referring to the drawing, a train braking system 10 incorporates an aircompressor 11 and main air reservoir 12 which maintains the air pressurein a train brake pipe 13 at a value above atmospheric pressure, forexample at 7 atmospheres (700 kPa). Air from the main reservoir 12 issupplied to the train brake pipe 13 via a pressure reducing valve 14,and hence via a check valve 15 to a brake reservoir 16 in each vehicleof the train. In each vehicle, air from the brake reservoir 16 issupplied to brake cylinders 18 (only one is shown) through a brake pipe19, there being a pair of brake cylinders 18 for each wheel set in thevehicle, the air supply being controlled by the driver by means of acontroller 20. Typically the controller 20 adjusts the brake pressure insteps—for example slight, moderate, and high braking, up to maximum(emergency) braking.

The system 10 also includes, in each vehicle, a wheel slide protection(WSP) system 22 which consists of a WSP valve 23 in the brake pipe 19 toeach wheel set, whose operation is controlled by a computer 24. The WSPvalve 23 effectively separates the pipe 19 into an inlet pipe 25 and anoutlet pipe 26. The valve 23 can be in one of three states: in one statethe air pressure in the inlet pipe 25 is supplied to the outlet pipe 26(and so to the brake cylinder 18); in a second state the inlet pipe 25is closed off, while the outlet pipe 26 is open to the atmospherethrough a choke; and in the third state both the inlet pipe 25 and theoutlet pipe 26 are closed off. The computer 24 receives input dataindicating the rotation rate of each wheel, and calculates the speed ofthe vehicle, and hence determines if a wheel is slipping. If thisoccurs, then the computer 24 controls the WSP valve 23 in the pipe 19 tothe brake cylinders 18 for that wheel set; the WSP valve 23 changes tothe second state so the pressure supplied to the brake cylinders 18gradually decreases. The degree of slip may be monitored at frequentintervals, for example every 30 ms, and each WSP valve 23 operatedaccordingly. If the degree of slip is acceptable then the WSP valve 23changes to the third state so the pressure in the brake cylinder 18remains constant. If the wheel stops slipping, then the WSP valve 23changes to the first state so the pressure in the brake cylinder 18 isagain increased.

Such wheel slide protection systems 22 are known. They come into effectonly if wheel slip occurs, and under those circumstances the brakepressure is modulated as described, so that the wheel turns at a rateabout 10 or 15 percent less than that required by the speed of thetrain. This optimises the braking effect.

The system 10 also incorporates, in the front vehicle of the train, apressure sensor 30 which measures the pressure in the outlet brake pipe26 and hence in the brake cylinder 18. This is preferably in the pipe 26for the leading wheel set 31. If the wheels are slipping, the pressureso measured provides a measure of the adhesion between the front wheels31 and the rail 40. An electrical signal provided by the sensor 30 issupplied to a sander control unit 32, which may operate a display 33 inthe driver's cab, for example if the adhesion is exceptionally low.

The sander control unit 32 also receives electrical signals indicatingthe brake level demanded by the driver, and signals from the WSPcomputer 24 indicating the calculated vehicle speed and the presence ofwheel slip. The control unit 32 hence calculates the amount of sandneeded to achieve the required deceleration. This is output as a controlsignal to a sand control valve 34 connected pneumatically to two sandhoppers 35 (only one is shown) mounted each side of the front vehiclejust in front of the third wheel set 36. The air pressure supplied tothe sand control valve 34 is that of the main reservoir 12. Each hopper35 includes a dispensing nozzle 38 through which dry sand is ejectedonto the rail 40 just in front of the wheels 36, at a variable ratewhich depends upon the signals received from the sander control unit 32.

Thus when the driver wishes to slow or stop the train he applies airpressure to all the brake cylinders 18 at a pressure determined by thecontroller 20. If any wheel set starts to slide, then the wheel slideprotection system 22 reduces the pressure in the brake cylinder 18 ofthat wheel set to control the amount of slip, and provides a signalindicating wheel slip to the sander control unit 32. If the front wheelset 31 of the train is slipping, a signal is provided to the sandercontrol unit 32 by the pressure sensor 30 indicating the degree ofadhesion. If the adhesion is below a threshold value, then the hoppers35 are activated to dispense sand onto the interface between the thirdwheel set 36 and the rail 40. This will improve the adhesion for thewheel set 36, and for all subsequent wheel sets in the train, as thesand tends to clean the rail head as the wheels roll over it. The lowerthe value of adhesion (as indicated by the sensor 30), the more sand isdispensed. The rate of dispensing of sand is also adjusted in accordancewith the speed of the train. Hence, if the ideal level of adhesionimprovement is achieved, the third and subsequent wheel sets will stopslipping. However as the leading wheel set 31 is unsanded, this willstill slip and act as a detector of low adhesion, so ensuring that sandis still applied whilst low adhesion conditions are detected on therails 40.

It will be appreciated that an apparatus for enhancing the wheel/railadhesion for a railway vehicle may differ from that described abovewhile remaining within the scope of the invention. In particular thepressure in the brake cylinder 16 might be calculated by the computer 24of the corresponding wheel slide protection system 22, rather than beingmeasured by a pressure sensor 30.

An alternative embodiment does not require the pressure sensor 30, andis applicable to vehicles which do not incorporates a wheel slideprotection system 22; in other respects the system incorporates thefeatures described above. In this embodiment the sander control unit 32receives signals from an accelerometer (not shown) which may beincorporated within the sander control unit 32. As described earlier,the sander control unit 32 receives electrical signals indicating thebrake level demanded by the driver. This brake level may be continuouslyvariable, or may be adjustable in steps, for example slight braking,moderate braking, high braking, and emergency braking; these.stepstypically correspond to decelerations of 0.3, 0.6, 0.9, and 1.2 m/s²respectively if the vehicle does not slip. The sander control unit 32determines if the deceleration measured by the accelerometer differsfrom the demanded deceleration by an amount greater than a thresholdvalue. Any such discrepancy between the values of deceleration indicatespoor adhesion, and the greater the discrepancy the lower is theadhesion. The sander control unit 32 may operate a display 33 in thedriver's cab, for example if the adhesion is exceptionally low.

The sander control unfit 32 can hence calculate the amount of sandneeded to achieve the demanded deceleration in accordance with theobserved adhesion. As described above in relation to the system 10, thisis output as a control signal to a sand control valve 34. The rate atwhich sand is ejected onto the rail 40 may be continuously variable, ormay be variable in steps. For example sand might be ejected at rates of0.5, 1.0, 1.5 or 2.0 kg/min in accordance with the demandeddeceleration, and the discrepancy between the values of deceleration.

Particularly at low speeds, the application of sand onto the rail 40 mayincrease the electrical resistance between the rail 40 and the wheels tosuch an extent that the track circuit does not detect the train. Thisproblem may be mitigated by reducing the rate at which sand is dispensedat such low speeds. Alternatively or additionally a sensor may beprovided to monitor the track circuit current through the axle of awheel set behind the sander hoppers 35, and reducing the sand dispensingrate if the current becomes too low. A suitable sensor for this purposeis described in GB 99 08599.5.

It will be appreciated that the system for enhancing the wheel railadhesion as described to improve railway braking performance may also beused to enhance traction performance. Wheel spin in traction may also bedetected by the wheel slide protection system 22 and signalled to thesander control unit 32, so that an appropriate amount of sand may bedeposited to increase tractive effort whilst minimising the risk to thetrack circuit. The sand flow rates may be adjusted according to trainspeed, the effect on track circuits, or the traction motor current onelectrical powered stock. This latter current acts as an indicator ofthe adhesion level in a similar way to the brake pressure describedearlier.

What is claimed is:
 1. A method for enhancing wheel/rail adhesion for arailway vehicle, the method comprising the steps of: monitoringoperation of the braking system when the brakes are applied, hencedetermining the a value of the adhesion, and applying adhesion-enhancingparticles to the wheel/rail interface at a rate which is adjusted inaccordance with the value of the adhesion so determined; the value ofthe adhesion being determined in at least one of the following ways: a)the vehicle incorporates an accelerometer, and the method comprisescomparing the deceleration measured by the accelerometer to thedeceleration demanded by the driver, and determining a value of theadhesion from any discrepancy between those values of deceleration; b)the vehicle incorporates a wheel slide protection system that modulatesthe pressure in a brake cylinder in response to detection of wheel slip,and a value of the adhesion is determined from a determination of thepressure supplied to the brake cylinder.
 2. A method as claimed in claim1 wherein the value of the adhesion is determined from a determinationof the pressure supplied to a brake cylinder, and wherein the particlesare applied to a wheel/rail interface that is behind the wheel on whichthe said brake cylinder acts.
 3. A method as claimed in claim 1 alsocomprising the step of providing a warning indication if the value ofthe adhesion is below a threshold value.
 4. A method as claimed in claim1 wherein the adhesion-enhancing particles are supplied dry.
 5. A methodas claimed in claim 1 in which the rate at which adhesion-enhancingparticles are applied is also adjusted in accordance with the speed ofthe vehicle.
 6. A method as claimed in claim 1 in which theadhesion-enhancing particles are applied at a rate that is adjusted toone of a plurality of different non-zero values.
 7. An apparatus forenhancing wheel/rail adhesion for a railway vehicle, the apparatuscomprising means to determine a value of the adhesion by monitoringoperation of the braking system when the brakes are applied, and meansfor applying adhesion-enhancing particles to the wheel/rail interface,at a rate which is adjusted in accordance with the value of the adhesionso determined in at least one of the following ways: a) the vehicleincorporates an accelerometer, and the adhesion-determining meanscomprises means to compare the deceleration measured by theaccelerometer to the deceleration demanded by the driver, and todetermine a value of the adhesion from any discrepancy between thosevalues of deceleration; b) the vehicle incorporates a wheel slideprotection system that modulates the pressure in a brake cylinder inresponse to detection of wheel slit, and the adhesion-determining meansdetermines the adhesion from a determination of the pressure supplied tothe brake cylinder.
 8. An apparatus as claimed in claim 7 in which theparticle-applying means adjusts the rate of supply in steps.
 9. Anapparatus as claimed in claim 7 wherein the adhesion-determining meansdetermines the adhesion from a determination of the pressure supplied toa brake cylinder, and wherein the particle-applying means applies theparticles to the interface between the rail and a wheel that is behindthe wheel on which the said brake cylinder acts.